J S S

ISSN 1615-9306 · JSSCCJ 38 (11) 1813–2006 (2015) · Vol. 38 · No. 11 · June 2015 · D 10609

JOURNAL OF

SEPARATION SCIENCE

Methods Chromatography · Electroseparation Applications Biomedicine · Foods · Environment

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J. Sep. Sci. 2015, 38, 1969–1976

Jing Yang1,2 Jun-qin Qiao2 Shi-hai Cui1 Jia-yuan Li2 Jin-jin Zhu2 He-xing Yin2 Cheng-yan Zhan2 Hong-zhen Lian2 1 Jiangsu

Collaborative Innovation Center of Biomedical Functional Materials, Jiangsu Key Laboratory of Biomedical Materials, College of Chemistry and Materials Science, Nanjing Normal University, Nanjing, China 2 State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry & Chemical Engineering and Center of Materials Analysis, Nanjing University, Nanjing, China Received February 8, 2015 Revised March 18, 2015 Accepted March 18, 2015

Research Article

Magnetic solid-phase extraction of brominated flame retardants from environmental waters with graphene-doped Fe3 O4 nanocomposites Graphene-doped Fe3 O4 nanocomposites were prepared by a solvothermal reaction of an iron source with graphene. The nanocomposites were characterized by transmission electron microscopy, atomic force microscopy, X-ray diffraction, superconducting quantum interference, Raman spectroscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. This nanomaterial has been used as a magnetic solid-phase extraction sorbent to extract trace brominated flame retardants from environmental waters. Various extraction parameters were optimized including dosage and reusability of the nanocomposites, and pH of sample matrix. The reliability of the magnetic solid-phase extraction protocol based on graphene-doped Fe3 O4 nanocomposites was evaluated by investigating the recoveries of 2,4,6-tribromophenol, tetrabromobisphenol A, 4-bromodiphenyl ether, and 4,4ʹ-dibromodiphenyl ether in water samples. Good recoveries (85.0–105.0%) were achieved with the relative standard deviation ranging from 1.1–7.1%. Moreover, it is speculated from characterization and magnetic solid-phase extraction experiment that there is not only ␲–␲ stacking but also possible hydrophobic interaction between the graphene-doped Fe3 O4 nanocomposites and analytes. Keywords: Brominated flame retardants / Graphene / Hydrophobic interactions / Magnetic nanocomposites / Solid-phase extraction DOI 10.1002/jssc.201500167



Additional supporting information may be found in the online version of this article at the publisher’s web-site

1 Introduction Brominated flame retardants (BFRs) are a group of chemicals based on bromine, which have been often added to textile, plastics, and electronic products to improve their fire Correspondence: Hong-zhen Lian, State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry & Chemical Engineering and Center of Materials Analysis, Nanjing University, 22 Hankou Road, Nanjing, China E-mail: [email protected] (H.Z. Lian) Fax: +86 25 83325180

Abbreviations: AFM, atomic force microscopy; BDPE, 4-bromodiphenyl ether; BFR, brominated flame retardant; DBDPE, 4,4ʹ-dibromodiphenyl ether; d-SPE, dispersive solid-phase extraction; Fe3 O4 /G, graphene-doped Fe3 O4 nanocomposites; Fe3 O4 /GO, graphene oxide doped Fe3 O4 nanocomposites; HBCD, hexabromocyclododecane; logKow , octanol–water partition coefficient; MSPE, magnetic solidphase extraction; PAH, polycyclic aromatic hydrocarbon; PBDE, polybrominated diphenyl ether; PBP, pentabromophenol; PMME, polymer monolith microextraction; SQUID, superconducting quantum interference device; TBBPA, tetrabromobisphenol A; TBP, 2,4,6-tribromophenol; VSM, vibration sample magnetometry; XPS, X-ray photoelectron spectroscopy; XRD, X-ray diffraction  C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

retardancy [1, 2]. The most widely used BFRs in the world were tetrabromobisphenol A (TBBPA), polybrominated diphenyl ethers (PBDEs), and hexabromocyclododecane (HBCD), while others such as tribromophenol (TBP) and pentabromophenol (PBP) were also used occasionally [3]. Recently, there is increasing concern about the potential adverse effects of BFRs on human health and the environment due to their high lipophilicity, resistance to degradation, and bioaccumulative properties [4–8]. The determination of BFRs in environment is important and has become a challenge which leads to the development of analytical methods with high selectivity, sensitivity, and reliability. Sample pretreatment is often used in the micro- or trace analysis of pollutants in complex environmental samples. Strategies based on solid-phase adsorbents, such as SPE [9], SPME [10, 11], dispersive solid-phase extraction (d-SPE) [12], and polymer monolith microextraction (PMME) [13,14], have been widely exploited because of their strong separation capacity, high enrichment factor, minimal sample and solvent consumption, low cost, and easy automation [15]. During the past decades, a new mode of SPE, magnetic solid-phase extraction (MSPE) has gained special attention in trace analysis [16–18]. Compared with traditional SPE, MSPE adsorbents combine numerous advantages such as large surface area, www.jss-journal.com

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unique magnetic property, convenient functional modification, simple manipulation, high separation efficiency, high reusability, and environmental friendliness. Since carbon materials are well known for their high adsorption capacity for organic compounds, many research groups have explored activated carbon [19], amorphous carbon [16, 20, 21], C8 [22], C18 [23], graphene [24–26], and graphene oxide [27] as outside coating materials of magnetic nanoparticles. Graphene (G), a novel class of carbon-based nanomaterial, is a single layer of carbon atoms. It has attracted considerable research interest because of its extraordinary electronic, thermal, and mechanical properties. Based on these remarkable properties, it has promising applications in many areas, such as molecular probes [28, 29], electrochemical sensors [30–33], and nanocomposites [34, 35]. Since the large delocalized ␲-electron system of graphene can form a strong ␲–␲ stacking interaction with benzene ring, it might be also a good candidate as an adsorbent for the extraction of benzenoid form compounds [27,36,37]. However, because of its extremely small particle size, it is difficult to isolate graphene from a solution phase after adsorption by traditional centrifugation and filtration. This restricts the applications of graphene in the extraction of pollutants in environment. In recent years, the development of magnetic adsorbents has been avoiding the problem due to the convenient magnetic separation after adsorption. Fe3 O4 nanoparticles are most frequently used as magnetic core owing to their easy preparation and good biocompatibility. Wang et al. prepared graphenebased magnetic nanocomposites for the extraction of pesticides [24, 38–41], herbicides [37, 42], phthalates [43–45], fungicides [46, 47], and polycyclic aromatic hydrocarbons (PAHs) [48]. Feng et al. [49] synthesized Fe3 O4 @SiO2 /G and applied it in the adsorption of sulfonamide antibiotics from water samples. Ding et al. [27] prepared graphene oxide doped Fe3 O4 nanocomposites (Fe3 O4 /GO) and investigated its availability in MSPE of PAHs from environmental samples. Up to now, however, there have been few works about the application of Fe3 O4 /G nanocomposites in MSPE of BFRs. In this paper, we employed magnetic Fe3 O4 /G as adsorbent for effective MSPE of 2,4,6-tribromophenol (TBP), TBBPA, 4-bromodiphenyl ether (BDPE), and 4,4ʹ-dibromodiphenyl ether (DBDPE) from environmental waters. Extraction conditions including sorbent dosage, reusage times, solution pH, and enrichment ability were optimized comprehensively. HPLC with UV detection was combined with the MSPE method to quantitatively determine the concentration of these BFRs analytes.

2 Materials and methods 2.1 Materials and reagents Graphite powder (ࣙ 98%) was supplied by Guangnuo Chemical Technology Company (Shanghai, China). TBP, TBBPA, (BDPE), and (DBDPE) (ࣙ 98%) were obtained from Sigma– Aldrich (Shanghai, China). HPLC-grade acetonitrile was purchased from Fisher Scientific (Fair Lawn, NJ, USA). Other  C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

reagents (all analytical reagents) were purchased from Nanjing Chemical Regent Company (Nanjing, China). Purified water (Wahaha Group, Hangzhou, China) was used throughout the experiments. Snow water was collected in our downtown campus in Gulou District, Nanjing. Tap water was taken from our laboratory in the downtown campus. All water samples were filtered through 0.45 ␮m membranes and analyzed within 24 h.

2.2 Apparatus The TEM images were obtained on a JEM-200CX microscope (JEOL, Tokyo, Japan). Atomic force microscopy (AFM) measurement was performed with a Molecular Imaging PicoPlus AFM (Agilent, Palo Alto, CA, USA) in an AC mode. The X-ray diffraction (XRD) patterns were collected on a XRD-6000 diffractometer with CuK␣ radiation (Shimadzu, Kyoto, Japan). The magnetism measurement was carried out using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design, San Diego, CA, USA). FTIR spectra were recorded on a Tensor 27 spectrometer (Bruker, Saarbr¨ucken, Germany). Raman spectra were recorded on a Labram HR 800 spectrometer (Jobin Yvon, Paris, France). The XPS analysis was performed on a PHI 5000 VersaProbe system (ULVAC-PHI, Kanagawa, Japan). The concentrations of all tested BRFs were quantitatively analyzed by HPLC on an Agilent 1200 equipped with a diode array detector (DAD; Agilent, Palo Alto, CA, USA).

2.3 Preparation of Fe3 O4 /G nanocomposites The preparation of graphene and Fe3 O4 /G magnetic nanocomposites referred to the previously reported procedures [50, 51] with minor modification as described in the Supporting Information. Meanwhile, pure Fe3 O4 nanoparticles were synthesized following a previous method [18] for purpose of comparison in the characterization of Fe3 O4 /G nanocomposites.

2.4 MSPE experiments For MSPE of BRFs from real water sample, 1 mL of 1.0 mol L−1 ammonium acetate-acetic acid buffer solution and 100 mL water sample were transferred into a beaker to obtain the sample solution with pH 5.0. Then, 25 mg of the prepared Fe3 O4 /G magnetic nanocomposites were added into the beaker and the mixture was stirred vigorously for 10 min to make the sorbents dispersed uniformly in the solution. Subsequently, an Nd-Fe-B magnet was put under the bottom of the beaker to attract the sorbents from the solution. The solution became limpid and the upper clear liquid was decanted 10 min later. After drying by air, the adsorbed target compounds on Fe3 O4 /G were eluted with 2 mL acetonitrile. After sonication and magnetic separation for 2 min each, the supernatant was filtered through a 0.45 ␮m membrane www.jss-journal.com

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Figure 1. Schematic illustration of synthesis of Fe3 O4 /G and MSPE of BFRs procedures.

for HPLC analysis of TBP, TBBPA, BDPE, and DBDPE. The synthesis and MSPE scheme is illustrated in Fig. 1.

2.5 HPLC analysis The chromatographic column used for HPLC analysis was a TSKgel ODS-100Z, 5 ␮m, 150 mm × 4.6 mm i.d. (TOSOH, Tokyo, Japan) at 30⬚C. A mobile phase of acetonitrile/0.1% perchloric acid solution (80:20, v/v) was employed at a flow rate of 1.0 mL/min. The injection volume was 10 ␮L. The UV detection wavelength was 235 nm.

3 Results and discussion 3.1 Characterization of Fe3 O4 /G magnetic nanocomposites 3.1.1 TEM The morphology of the as-prepared Fe3 O4 nanoparticles, graphene, and Fe3 O4 /G nanocomposites were characterized by TEM analysis. As shown in Supporting Information Fig. S1-A, the TEM image of Fe3 O4 revealed that the nanoparticles were spherical in shape with a mean diameter of approximately 120–200 nm. The graphene sheet with an irregular shape was ribbon-like and contained some wrinkles, which maintained a large surface area (Supporting Information  C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Fig. S1-B). Supporting Information Fig. S1-C presents the representative TEM image of the prepared Fe3 O4 /G nanocomposites. It can be observed that the Fe3 O4 nanoparticles were homogeneously anchored onto the surface of the graphene sheets [49]. 3.1.2 AFM The preparation of graphene and Fe3 O4 /G nanocomposites were also investigated by AFM image in Supporting Information Fig. S1-D. The morphology of graphene exhibited a lateral extent of a few hundred nanometers [49,52]. Based on the typical AFM image, the height of the graphene sheet is about 1.0 nm, indicating it has single- or few-layer structure. Moreover, the white spots indicated that the Fe3 O4 nanoparticles were distributed on the graphene layer. As shown in the crosssectional view of the AFM image, the typical height of the nanoparticles over graphene surface was about 7.5 nm. The observation confirmed that Fe3 O4 nanoparticles have been successfully decorated onto the graphene sheets. 3.1.3 XRD XRD measurements were performed to obtain crystalline structural information for the synthesized graphene and Fe3 O4 /G nanocomposites. The XRD pattern of graphene (Supporting Information Fig. S2-A) showed a broad peak at 2␪ = 26.2⬚, corresponding to the (002) reflection of graphene [38, 51]. From the XRD pattern of Fe3 O4 /G (Supporting www.jss-journal.com

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Information Fig. S2-B), the diffraction peak of graphene was weakened greatly. The significant diffraction peaks of Fe3 O4 /G matched well with the data from JCPDS card (19– 0629) for Fe3 O4 with the diffraction angles (2␪) equal to 30.25, 35.58, 43.21, 54.39, 57.09, 62.92, and 75.19⬚. The results indicated that the decoration of graphene did not change the crystal phase of Fe3 O4 in Fe3 O4 /G nanocomposites. 3.1.4 VSM The vibration sample magnetometry (VSM) curves of Fe3 O4 nanoparticles and Fe3 O4 /G nanocomposites at room temperature are given in Supporting Information Fig. S3. They both exhibited typical superparamagnetic behavior with the maximal saturation magnetizations of 80.2 and 59.9 emu/g, respectively. The decrease of the maximal saturation magnetization of Fe3 O4 /G may result from the involvement of non-magnetic graphene sheets. When an external magnetic field was applied, the Fe3 O4 /G nanocomposites can be attracted to the wall of the glass bottle in a short time, as shown in the right-bottom inset of Supporting Information Fig. S3. 3.1.5 Raman spectroscopy Raman spectroscopy is a powerful, nondestructive tool to characterize carbonaceous materials because of their high Raman intensities. Supporting Information Figs. S4-A and B show the Raman spectra of Fe3 O4 nanoparticles and Fe3 O4 /G nanocomposites, respectively. The signal at 591 cm−1 was in accordance with Fe3 O4 particles [53]. The peaks at 1592 and 1350 cm−1 corresponded to G and D bands of graphene, respectively. The G band is attributed to the vibration of sp2 bonded carbon, whereas the D band is related to the disorder or structural defects of graphene [54]. 3.1.6 FTIR spectroscopy Supporting Information Figure S5 exhibits the FTIR spectra of graphene oxide (A), graphene (B), and Fe3 O4 /G nanocomposites (B). The spectra B and C both had C=C and C–C stretching vibration adsorption peaks at about 1600 and 1200 cm−1 , respectively, which showed the feature of graphene structure. Based on the result of Raman spectra, the presence of two carbonaceous functional groups implied that Fe3 O4 /G nanocomposites surface possess not only highly delocalized conjugate ␲-electrons but also moderate hydrophobic sites, which are apt to predominantly attract benzenoid substances by ␲–␲ stacking and hydrophobic interactions, respectively [18, 55]. In Supporting Information Figs. S5-B and C, the peaks at 1720 and 1100 cm−1 ascribed to C=O and C–O stretching vibration, respectively, were too small to distinguish in reference to those of GO (Supporting Information Fig. S5-A) due to the hydrazine hydrate-assisted chemical reduction of GO to graphene [55]. In Supporting Information Fig. S5-C, the Fe–O characteristic stretching vibration peak at 580 cm−1 was still very distinct, proving that Fe3 O4 nanoparticles were successfully anchored onto graphene sheets [56].  C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The O–H peak at 3000–3700 cm−1 was also observed, which may be caused by the side-reaction in solvothermal synthesis of iron resource and graphene with ethylene glycol as the solvent, or the absorption of water onto the nanocomposites. 3.1.7 XPS The chemical composition of Fe3 O4 /G nanocomposites was further investigated by XPS. The wide scan spectrum of Fe3 O4 /G shown in Supporting Information Fig. S6-A indicated the existence of carbon, oxygen, and iron in the composites. The photo electron lines at binding energies of approximately 280, 530, and 710 eV were attributed to C1s , O1s , and Fe2p , respectively. Supporting Information Fig. S6-B gives the C1s spectrum of Fe3 O4 /G. As expected, two carbonaceous groups, C=C (284.5 eV) and C–C (285.6 eV), were observed, indicating the assembly of graphene on surface of Fe3 O4 /G nanocomposites [50]. It is well known that graphene is oneatom-thick 2-D layers of sp2 -bonded carbon [55]. Following Raman and FTIR spectra, XPS spectra further confirmed the existence of C=C and C–C groups, supporting the occurrence of ␲–␲ stacking and hydrophobic interactions between Fe3 O4 /G nanocomposites and analytes. Meanwhile, the peaks at 286.4 and 288.4 eV respectively indicated the existence of C–O and C=O, which also probably originate from the byproducts of the solvothermal reaction. In comparison with GO, graphene sheets have poor solubility in solvents, owing to the lack of proper functional groups on its surface [55]. However, the oxygen containing groups partially decorated on the nanocomposites endowed Fe3 O4 /G nanocomposites with weak hydrophilic property, facilitating the uniform and stable dispersion of the nanocomposites in aqueous solution, and in some cases producing weak dipole–dipole or hydrogen bonding interaction between the nanocomposites and target analytes [18].

3.2 Optimization of MSPE conditions In this work, TBP, TBBPA, BDPE, and DBDPE were selected as test analytes to evaluate MSPE efficiency of the prepared Fe3 O4 /G nanocomposites for BFRs. The initial concentration of each BFR was 0.04 mg/L. The concentrations of these BFRs after the MSPE were determined by HPLC according to the procedure in Section 2.5. To realize the best extraction performance of BFRs on Fe3 O4 /G nanocomposites, a variety of parameters such as amount of the sorbent, pH of sample matrix and reusability of the sorbent were optimized in detail. 3.2.1 Dosage of the sorbent The amount of magnetic sorbent was optimized by varying Fe3 O4 /G nanocomposites from 5 to 50 mg. It can be found that with the increase of Fe3 O4 /G dosage, the recoveries of four analysts increased and reached maximum value when 20 mg of Fe3 O4 /G was used (Fig. 2(A)). In all the following www.jss-journal.com

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Figure 2. A. The recoveries of BFRs on different amounts of Fe3 O4 /G nanocomposites sorbent (n = 3); B. The dependence of recoveries of BFRs on pH (n = 3); C. The reusability of Fe3 O4 /G nanocomposites sorbents. The concentrations of the analytes were all 0.04 mg/L.

experiments, therefore, 25 mg Fe3 O4 /G nanocomposites were adopted to assure complete adsorption of BFRs. 3.2.2 pH of sample matrix The pH of aqueous solutions was controlled using 1.0 M ammonium acetate buffer solution with different pH. The pH  C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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of the buffer solution from 3.3 to 10.0 was adjusted by acetic acid or ammonia. Fig. 2(B) illustrated that the dependence of recoveries of the four analytes upon pH was very different. BDPE and DBDPE do not dissociate, therefore, no obvious variation for their MSPE performance was found in the whole pH range tested. TBP (pKa = 5.95 [57]) and TBBPA (pKa1 = 7.7±0.4, pKa2 = 8.5±0.4 [ACD/Labs Website; https://ilab.acdlabs.com. Algorithm Version: v5.0.0.184]) have hydroxyl group(s) in their structures. In acidic matrix, they can exist in molecular status and the recoveries were higher than 80%. With the increase of pH, however, the extraction efficiency decreased significantly for these two ionizable analytes. This may be attributed to the decreased hydrophobic interaction of dissociated TBP and TBBPA with graphene anchored on Fe3 O4 /G nanocomposites. However, the recoveries TBP and TBBPA reached 80% once pH was below 5.0. Consequently, the following experiments were conducted at pH 5.0 to protect the service life of Fe3 O4 /G although the extraction efficiency was even higher at more acidic medium. From the results in Figs. 2(A) and 2(B), it is found that the order of recovery is BDPE > TBBPA > DBDPE > TBP. It is generally known that the absorption mechanism of graphene is ␲–␲ stacking interaction. TBP possesses only one benzene ring and the least delocalized ␲-electrons. Its recovery was the lowest among these four tested BFRs because its ␲–␲ stacking interaction towards graphene on Fe3 O4 is weaker than that of TBBPA, DBDPE, and BDPE which have two benzene rings. Moreover, BDPE and DBDPE possess an oxygen atom between two benzene rings, leading to stronger conjugation effect due to the participation of an additional lone electron pair from oxygen atom in the system. BDPE was more easily extracted than DBDPE. This is likely related to the smaller space effect of one Br in BDPE than two Br atoms in DBDPE. There is a carbon atom between two benzene rings of TBBPA, not increasing its conjugation effect, and TBBPA should be more difficult to extract than BDPE and DBDPE. However, the recovery of TBBPA was not lower than DBDPE. This is probably because the hydrophobicity of TBBPA is stronger than DBDPE, as well as the spatial resistance of an isopropyliden between two benzene rings. Interestingly, the recovery order can also be explained in the view of hydrophobic interaction [18]. The recovery of TBP was the lowest. This is because its octane–water partition coefficient (logKow = 4.404±0.481) is smaller than that of TBBPA (logKow = 9.693±0.698), BDPE (logKow = 5.037±0.407), and DBDPE (logKow = 5.937±0.497) [58]. The dissociation of phenolic hydroxyl group in TBP in weak acidic medium is another cause for the weakest absorption. TBBPA has the highest hydrophobicity. However, its recovery was lower than BDPE due to its partial dissociation at pH ࣙ 5.0. DBDPE is more hydrophobic than BDPE. The reason that the recovery was lower than BDPE is also the space effect of Br atom. It is reasonably speculated that, as well as ␲-␲ stacking interaction, hydrophobic interaction participates in the SPE of Fe3 O4 /G nanocomposites, which accords well the above result about the influence of pH on extraction performance of analytes. www.jss-journal.com

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Table 1. Recoveries of BFRs in real water samples (n = 3)

Water sample

Tap

Snow

Spiked / mg/L

Detected / mg/L

0 0.02 0.10 0 0.02 0.10

Recovery /% (RSD /%)

TBP

TBBPA

BDPE

DBDPE

nd 0.018 0.091 nd 0.017 0.092

nd 0.019 0.097 nd 0.019 0.096

nd 0.020 0.100 nd 0.021 0.104

nd 0.019 0.096 nd 0.019 0.095

TBP

TBBPA

BDPE

DBDPE

90.0 (1.1) 91.0 (2.2)

95.0 (3.2) 97.0 (3.3)

100.0 (4.6) 100.0 (7.1)

95.0 (1.5) 96.0 (3.7)

85.0 (4.6) 92.0 (1.3)

95.0 (3.8) 96.0 (1.6)

105.0 (3.3) 104.0 (5.5)

95.0 (5.9) 95.0 (2.6)

Table 2. Comparison of the present work with other pretreatment methods for determination of BFRs by HPLC-UV

Methods

Analytes

Sample volume / mL

Pretreatment time / min

LODs TBP

SPME PMME MSPE MSPE

River water and wastewater Tap, lake and rain waters Tap, lake, rain and snow waters Tap and snow waters

TBBPA

BDPE

RSDs /%

Recovery /%

Ref.

5.1

103.6

[10]

DBDPE

10

32

0.9

2.4

45

0.2

0.15

0.1

1.3–4.4

78.7–106.1

[13]

100

15

0.5

0.4

0.3

0.3–6.8

80.0–110.0

[59]

100

15

0.4

0.3

0.5

1.1–7.1

85.0–105.0

This work

0.2

3.3 Analysis of environmental waters

Figure 3. HPLC–UV chromatograms of snow water (A), snow water spiked with 0.02 mg/L (B) and 0.1 mg/L (C) of each analyte. Detection wavelengths: 235 nm.

3.2.3 Reusability of the sorbent The used magnetic sorbents were washed several times with water and ethanol in turn, and then dried for reuse. There was no obvious reduction in MSPE recoveries of TBP, TBBPA, BDPE, and DBDPE after 30 runs using a batch of Fe3 O4 /G nanocomposites (Fig. 2(C)). The good repeatability indicated that this magnetic sorbent was stable and durable during the extraction procedure.  C 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

With the overall optimized MSPE procedure and HPLC–UV detection, the calibration curves established for TBP, TBBPA, BDPE, and DBDPE were summarized in Supporting Information Table S1, where x and y represent the concentration of BFRs and the corresponding peak area, respectively. Good linearities were achieved over the concentration range of 0.002–0.5 mg/L with the correlation coefficients (r) ranging from 0.9958 to 0.9999 for these BFRs. The LODs at S/N = 3 were 0.2–0.5 ␮g/L. The proposed method was applied to analyze TBP, TBBPA, BDPE, and DBDPE in different environmental water samples, including tap and snow waters (Table 1). To determine the availability of this method, the water samples were spiked with 0.02 and 0.1 mg/L of each analyte, respectively. The recoveries of four analytes fell in the range of 85.0–105.0% with the RSD range of 1.1–7.1%. A typical HPLC–UV chromatogram for snow water sample is shown in Fig. 3. The proposed method indicated a good accuracy, precision, and reliability, as well as simplicity and environmental friendliness.

3.4 Method comparison The extraction effects of TBP, TBBPA, BDPE, and DBDPE were enhanced greatly by the proposed MSPE coupled with HPLC–UV detection. This MSPE protocol can be evaluated by comparing with other reported works [10, 13, 59] from the www.jss-journal.com

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viewpoints of sample volumes, pretreatment times, LODs, RSDs, and recoveries (Table 2). It can be shown that the MSPE method based on Fe3 O4 /G nanocomposites is better than or comparable with other SPME [10], PMME [13], or MSPE [59] method. In general, this MSPE protocol is a satisfactory means to determine BFRs from environmental waters without troublesome centrifugation or filtration procedure. It may have a bright prospect in potential applications.

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4 Concluding remarks In this paper, superparamagnetic Fe3 O4 /G nanocomposites have been successfully used as MSPE sorbents for BFRs from large volumes of environmental water samples followed by the determination with HPLC–UV. The prepared nanocomposites were characterized by various methods such as TEM, AFM, XRD, VSM, Raman, FTIR, and XPS. A series of MSPE parameters for TBP, TBBPA, BDPE, and DBDPE were optimized comprehensively for reliable real-world application. It was demonstrated that this Fe3 O4 /G nanocomposites based MSPE–HPLC–UV protocol is a satisfactory analytical method for trace BRFs. The adsorption mechanism of this nanomaterial to the BFRs has been elucidated. It is concluded that ␲–␲ stacking interaction coupled with hydrophobic interaction dominates the retention behavior of Fe3 O4 /G nanocomposites. This work was supported by National Basic Research Program of China (973 program, 2009CB421601, 2011CB911003), National Natural Science Foundation of China (21275069, 21177061, 90913012), National Science Funds for Creative Research Groups (21121091), and Analysis & Test Fund of Nanjing University. The authors have declared no conflict of interest.

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